CA2440484C - Sag compensating device for suspended lines - Google Patents

Abstract

A device that automatically compensates for heat-induced sag in a suspended line by using a shape memory alloy actuator. As the actuator is heated, it contracts, producing a pulling force in the line, and thereby reducing sag.

Description

SAG COMPENSATING DEVICE FOR SUSPENDED LINES
FIELD OF INVENTION
This invention relates to devices for holding electrical transmission lines to each other and to towers and the like, and more specifically to such devices that can automatically compensate for changes in transnlission line sag.

nES-CRIMON OF THE PRIOR ART
Transmission power lines are electrical lines that typically carry high voltage, e.g., 230 KV. For reasons of safety, such lines are suspended well above ground level, typically finm towers or the like. li'IG.1 shows towers 10 and 20 that suspend a power line 30.
(Although in practice the towers will have extension arms and will carry several lines, for ease of illustration, FIG. I has been simplified.) Power line 30 is suspended from towers with insulating devices 40, for example ceramic or glass and rubber and fiberglass insulators whose length can range ftom a few inches, such as six inches, to over fifteen feet, such as twenty feet, depending on the voltage in the line and the environment.
Power lines, which are generally supported by transmission towers, cover large distances.
Due to the force of gravity, power lines intrinsically tend to sag. This initial sag increases with line temperature because the conducting material of which the line is made expands as line temperature increases, effectively lengthening the line. A small increase in line length produces a large, and potentially hazardous increase in sag. For example, for a line with a 500 foot tower spacing (a typical span for overhead transmission liues) and an slurnixtunm conductor stecl reinforced (ACSR) GonduGtor (drake), a temperature increase of a'baut 120 F (from IOO F
to 212 F - which can represent the expected conductor temporature oliange between winter and summer montbs) will causes about 6.4 iuohes increase in line length, which will inarease the sag by about 4.7 feet, For the purpose of this calculation, line tension at 44 F was set to 20 /a ofthe conductor breaking load (a common practice by the tra.nsmzssion line desi6mers).

Imorease in line temperature may be due to a number of factors including increased ambient air temperatuxe, decreased wind flow over the Ifne and increased cuxrent flow ttuongh the line. Sagging powei lines cteate :~ke ha7Ards and other pu,blic safety issues due to ground clearance. The cost of line sag in torxns of energy not sold and also t= ftimmisYg an,d. Iitigation expenses are very well known to the clectricity gemcltiQn and txa,llsI'L1i.ssioXl lndilsty.

FiSurE 1 illustrates line sag. The phantom-Iine version of power line 30, 30' represents the power line's position at a temperature of Ti, such that terapezaturo T1 is greater than temperatuxe To. The distance from the sagging line to the ground is represented byy. The extent of the sag is indicated by tYae distanoe Dy botween the power 1ine 30 and the phantom power line :30'. JET is the hei,gtt of the towen; and Dx is the span length Such sag can redluce the ciea.ranoe between ground and the trao,smissxor,s lines, The problem of sagging power lines is well known to the electri.o power indusUy and is associated with problems which are hazardous and which are both time conSUming and expensive to xactify. Sag&g power lines pase an eleotrooutzon hazard to persons and vehicles and can lead to interruption in power supply and are known to cause hugely destruative arzd expensive foxest and brush t~xes.

2 The same prablern of sag 4Iso affects all other suspended strucq:tres suoh as bridges, susp$nded telecoammxunications wires and stzvctmal cables. (not really for this ea,se!), Suoh wires and cables include cables used in consi:ruction of buiildings and bridges.
Additionally the same problem ms.y affect aty wire that is under tension, such a gu.ide wires and cab7cs used for lransniittiiig force from a control to an ixtstrument such as may be used in boats and aircxaft and cars s. ,d other mu,;hines tm, for example, control a rudder or aerolon 6r braking system.

Presen,t teclmiques to compensate fox stteh sag-caused by undesired incmase in length of a cable include:

(i) Shortening the distance betwc= adjacer,t tQwers to reduce span lengfh and thus reduGE line sag; this is shown in figuxt 1A wherein be is.shoxtet than Dx and wherein Dy' is shorter than Dy.

(ii) Brectixtg taller transmission toNvexs to accommodate line sag; tkt,is is shown in figrue AB in whiah H' is gxeater than H and y is geater than y, (iii) Replacing exitiug conductors with new ones with h-ighez anapaeaty or lower sag chamcteristics.

(iv) Retro-$tting existing toweas to x7acrease height.

(v) Ti.irniring electiioal cunant load capacity to compensate tbx increased ambient temperature.

(vi) Othex rriethods for reducing sag and for %eeping a suspended line taught x;aclude thc use of constant fiension elemeiits su.ch as springs atrd pre-stressed tensiomers and even the use of steategically placed weights on the suspen.ded line.

3 (vii) Other methods for combating sag have been disclosed in previous patents to the inventor, Manuchehr Shirmohamadi: U.S. Pat. No. 6,057,508 and U.S. Pat. No.
5,792,983 and PCT US9917819, W00008275A1. These previous solutions automatically compensate for thermally induced line sag by exploiting the concomitant thermal expansion of a coupling member or actuating rod. But these previously disclosed solutions require an amplification means that includes one of the following: dual scissor arms (FIGS. 3, 4, 9, 10, 11) or a dual rotary system (FIG. 5) or a single rotary system (FIGS. 6, 7a, 7b, 7c) or a skewed gear system (FIGS. 8, 8b, gc). All of these previous solutions use a thermally-expanding actuating rod that produces compressive forces that are used to mitigate sag.
None of these previous solutions use a coupling member ifiat contracts, producing tensile forces that are used to reduce sag. These solutions, while effective, require a good deal of engineering and manufacturing effort. There is a need for B device that mitigates sag in a suspended line, that employs the principles of thermal expansion of an actuating rod, that is also simple and easy to manufacture and easy to install.
(viii) Certain other methods are disclosed in the Soviet patents and inventor certificates numbers 454627, 754541, and 974483. These old Soviet solutions are interesting in that they use a shape memory alloy (SMA) to tighten a line. Essentially it works by simply taking a length of SMA and attaching one end to a spot on the power line and the other end to another spot on the power line, so that the SMA lies approximately parallel to the power line. Current passes through the SMA, which shortens, tensioning the line.l3ut this approach has been tested and does not work. Firstly, beeause the power line is uncut, when the SMA tightens, a large hanging loop is formed below the position of the main line. This sag is itself; a problem. Secondly, the frequent flexing of the power line fatigues the line, which eventually breaks. Additionally, the Soviet devices are different from the present invention because: they do not cut the line, but simply make a"bubble" by attaching the SMA along the length of the uncut line; and unlike our invention, since they keep the line in tact, it wiIl be difficult to prevent the current passing through the

4 line, and not the SMA.; au.d algo *e 9oviet system employs no suagaffication;
and s'Eso otu invention b,4s a"safety linit"'that elfioiuatss meohsWcal lqad mrying by SMA at aoXd couditioua and produces am itysvmroe if the S1VIA. is dam~aged/broken~
for any reason.

Current in the line causes liue tempcratuxe to rise sad resistari.ce to increase - leaftg to stM fiuiher temparatwre inareaee. Tbe temperature incxease aauses the line to sag due to thermal expusion of the line. Thxs is particularly impordmt at tima ofpeafc demand such as during the hot days of summer wheu demand for air conditiommg is hxgh. Ifth mnbient t=peratahte, high sm radiarions, as wall as ambient conditions with low to no wind also conttibuta to line sag by increas;irg #he effactive ta~oape~uza of the lm.e, Seasonal changes in ambieait air 1mnpaature is also a signi~'tcatlt comtributor to hae sag. To compeusate far incroaseal ambient air tanVetatwe during warm snmmar xaontl,s, aleatric ufilitfes muat often mduce cmxent conducted by the powet lines at the very time when dam,snd Ytuay be h9ghest, e.$., to power air eonditio::ers.
Such ourrent limitaatlon may be in.thd 30% range dnft swmam monffis, w'hich msulis in ecawmiic losaea to ths power utility dtie tio iuability tD meet demaad.
Needless td ny, ezacting b,igher amy/oC more clos* spacxd towers mprwients aA.rthec' eoomomic cost eo the power utflity.

There is a need for a device tiaat can be used to rr4vte treaassnission power lines to carry greater amounts of eZectricityr, which oan actotnat9celly compensate fcr obanges in conductor tarnperature to mduee thornmai sag in the associated power linas.
Pre:fambIy sneh devioes should be inte.xpansive to fabricate, inexposive to iustall, and subata,ially mainftmce fte. ne present inverttion aisclosas sach a device, and a method for reducing sag caused by tonVerature inarease iu mispeuded elecbzaal ~0~1 v~+es,' _S-SCTMINARY OF 'I'HE IIWENTION
The Sagging Line Mitigator (SLiMTm) and the SmartConductorm automatically compensate for sag in a suspended or hanging line, such as a power line. Both use a material that changes its dimensions as a function of temperature. One such material is shape meinory alloy (SMA) which undergoes a phase transfonmation upon temperature change (referred to as transition) and produces a significant change in size and geometry.
In this invention, both SLiMFm and SmartConductorm use an SMA to conduct all, some or none of the total current in the power line. The SMA is heated by resistive heating (power loss=resistanceXcurrent^2) of the SMA or by conduction from the conductor which itself is undergoing temperatuue increase due to resistive heating caused by cun-ent or by a combination of both methods. As the temperature of the SMA changes, it goes through the transition and will change shape accordingly, In this invention, the SMA will contract as its temperature increases. The contraction of the SMA produces a pulling force (increasing tensile force) which is directly (for SrnartConductor7t"') or indirectly (for SLiM11N) usnsferred to the suspended line, effectively pulling in the siack and reducing sag. SLi1Vff uses at least one iever to atnplify the SMA length change and transfer it to the suspended line. The SmartConductorm does not use any lever and the length change of the SMA is applied to the suspended line directly, without any magnification.

The devices may be installed between the tower and the suspended line, or n,ay be installed within the span of the suspended line. Both devices may be installed using techniques similar to those used for installation of a "splice" or a "dead-end" on such lines.
The "splice" technique is achieved by cutting the line at two positions at a given distance from each other and installing the device by connecting the device ends to the cut ends of the line. In case of a "dead-end" technique, installation is achieved by cutting the power line at one location at a given distance from its end connecting point to a fixed structure, such as a tower, and installing the device between the cut location of the line and the fixed structure and connecting the ends ofthe device to the out end of the line and the fixed sttucture. ,A.lso, mulkiple deyiceican be 3nsto,lled in series if ncEded by cuttin,g longer pieces of the power line.
Many diffeTent types of shape meutory alloy (SMA) are lmoww~n, for example, cacnmon SMA's inolude the following:
NicxcelATitanium alloys - CoppexlZincJAlitrrnin,inra Alloys = CoppcxlAluminium,Nkke1 Alloys Other alloys that axe kuawn to display shape memory propertie$ are, Silver/Ca.dzIIiuna Alloys = Gold / Ca.dmiu= alloys Copper / Tin alloys = Ctrpper f Zinc alloys = Indiam / Titaniuxu alloys Nickel ! Alurninium alloys = Iron / Platinum atloys Manganese / copper alloys = Iron / Manganese 1 Silicon alloys A good reference book describing SMA is SShape Memory AlIoys' (CISMC
IrLtern.ational Centre fQx Mechanical $ciences: Courses and Lectures) Ivx, Fremond. & S. Miyazaka (Editors).

The invention may take many different embodinzents, some o~'whicb. are set out below, depending on the =angemaent of stav.otural elwnen.ts. Bu't eacb mbodiment does the same thing, n5i4a.tes line sag, in essetstia.iiy the same way, by reducing the e~fective length of a power line ftQu$h a direct or meohanica.lly arnpliiied change in the len,gth of a SMA Qr other materials wi'iich will ttndeyrga dirn,ension.a1 change with tem,perature change.

The objects and advantages of the zn.vention inol'ude,latit are not linlited toa (j) provision of a meanS of mitRgating power 13nc sag.Whxch is considerably less expensive tlan current means;

(ii) provision of a means of mitigating power line sag which is automatic and self-adjusting such that the same change in ambient conditions (temperature, wind speed and direction, and so)ar radiation) that causes the line to sag will concomitantly cause the invention to act to mitigate the line sag;
(iii) provision of a means of mitigating power line sag which is automatic and self-adjusting such that the same change in line current (ampacity) that causes the line to sag will concomitantly cause the invention to act to mitigate the line sag;
(iv) provision of a means of midgating power line sag without the necessity of to replacing the power line with a new one with higher current capacity or lower sag characteristics;
(v) provision of a means of mitigating power line sag without the necessity of doubling or tripling (bundling)) the power line with more conductors;
(vi) provision of a means of mitigating power line sag without the necessity of retrofitqng transmission towers to make them taller;
(vii) provision of a means of mitigating power line sag which will allow transmission towers to be spaced at greater intervals than is presently necessary, thereby necessitating the erecting of fewer transmission towers;
(viii) provision of a means of mitigating power line sag which will allow the building of shorter t'ransmission towerS than is presently necessary;
(ix) provision of a means of mitigating power line sag without reducing line current (ampacity);
(x) provision of a me.ans of mitigating power line sag which is inexpensive to manufacture and install, and is essentially maintenance-fi-ee.

According to a frrst aspect of the invention there is provided for reducing sag in a suspended cable, a sag-compensating device having a first end and a second end and comprising a shape memory alloy actuator disposed therebetween, wherein the actuator contracts as its temperature increases, and wherein the suspended cable is not strung continuously between the first end and the second end, and wherein at least one end is adapted to be connected, dirwtly or indirectly to the suspended cable, and wherein the other end is adapted to be connected, directly or indirectly to either another section of the suspended cable or to a fixed point, wherein as the temperature of the actuator increases, the actuator contracts, and a pulling force is applied to the cable by the actuator, reducing sag in the cable.

Other features and advantages of the invention will appear from the following description in which the prefcrred embodiments have been set forth in detail, in conjunction with the accompanying drawings.

FIG. 1 depicts a generic power line transmission system according to the prior art showing the problem of line sag;

FIG.1 A depiets a generic power line transntission system according to the prior art wherein the problem of line sag has been mitigated by shortening the distance between adjacent towers to reduce span length and thus reduce line sag;

FIG. IB depicts a power line transmission system according to the prior art wherein the problem of line sag has been mitigated by erecting taller transmission towers to accommodate line sag;

FIG. 2 depicts the SLiM'm device fully assembled as tested.

FIG. 3 depicts the SmartConductorm device fully assembled as tested.
FIG. 4 is an assembly drawing of the SLiM"" device showing components.

FIG. 5 is an assambly drawing of the SmartConductorm device showing components.

FIG. 6 depicts the Shape Memory Alloy actuator component for SLiMTM.
FIG. 7 depicts the set-up for functionality testing of the devices.

DESCRIPTION OF THE INVENTION
The inventor has invented a device for controlling and mitigating transmission line tension/sag.
S'agging Line Mitigator_( CfMTM) SLiMTm is a new class of transmission line hardware that fixes the high temperature sag problem by using a material which reacts to temperature change by significantly changing its own size and geometry, such as a shape memory alloy (SMA). SLiMTM reacts to increasing conductor temperature by decreasing the effective length of conductor in the span. The material component that affects the change in length is referred to as an "actuator." In one embodiment, the material used in the actuator is a Shape Memory Alloy (SMA) that shortens as :its temperature increases, and the change in length of the actuator produces a pull that is amplified and transfered to the transmission line using a single lever system. Other embodiments may use more than one lever to amplify the motion of the actuator onto the power line. The effect is a decrease in line sag during high temperature operation that may-be fiuthcr transmitted through several adjacent spans, depending on construction specifics.

How SLiMTM Functions SLiMTm uses a Shape Memory Alloy and is activated by the same temperature changes that cause a conductor to sag. The device is passive--there are no motors or electronic controls. As temperature increases, the SMA contraots and the SLiMT" device changes its geometry to apply a pull on the line thereby decreasing line length. As conductor temperature returns to normal, SLiMT''' returns to its original geometry. It does not need to be reset and is always ready to respond to the next conductor temperature fluctuation.
The actuator element (using shape memory alloy) of SLiMT"' forms part of the conductor, so that a part or all of the total current is conducted through the actuator when in use, The rest of the current is conducted through another element of the device such as the body, which may be formed of one or more hollow tubes. In certain embodiments, the SMA is surrounded by the hollow tube (pipe) body. The body in one embodiment acts both as a structural element for the lever action and as a housing to reduce the corona emitted by the device.

A SMA has a start transition temperature and a stop transition temmperature at which the physical change (transition) begins and ends. The amount of current required must be sufficient to cause a temperature increase such that the SMA experiences a full or partial transition from one shape or length to another. The temperature required to cause transition is a function of the SMA being used, its dimensions and properties, and may be measured by conducting various, and in most cases customary mechanical and electrical testing on the given SMA.

Sn~unConductorw The actuator in the SmartConductor""'' is also made of the similar material (in this case, shape memory alloy) as SLihfn". However, the actuator is wrapped inside an aluminum or other conductive materials similar to the construction of the overhead transmission lines, e.g. ACSS (Aluntinum Conductor Steel Supported) which has steel core cables wrapped by multiple layers of aluminum wires. The temperature increase on the SmartConductor'm actuator is primarily by direct heat transfer (conduction) from the aluminum cover which itself heats due to normal resistive heating. Temperature increase of the SrnartConductor""' will cause its SMA core to go through pardal or full transition and reduce the effective length of the SmartConductor' and hence the power line in the span.
SmartConductor'"
does not use any amplification system as does SLiM'', but simply reduces the length of the transmission line by an amount equal to the amount of shortening of the SmartConductorT"''. Despite the lack of amplification, the inventor has calculated that the conductor length reduction provided by SmartConductorT"' will be adequate for many applications. Furthermore, multiple or longer SmartConductors"'' can be placed on power lines to increase its effect on the line. Furthenmore, SmartConductorm may be manufactured as a new conductor for new installations or replacing existing lines of overhead transmission lines which will let the line operate at higher temperatures and lower sags than existing conductors such as ACSR or ACSS.

SmartConductorT '' is a simpler device than SLiMT"' (which itself is of considerably less complexity than previous systems) and may be manufactared and installed at a very cost-efficient price.

SmartConductorTM~ affects a decrease in line sag during the high temperature operation that maybe transmitted through several a4jacent spans, depending on construction specifics.

How SmartConductorw Functtorts SmartConductorlr"'' is activated by the same temperature changes that cause a conductor to sag--ambient conditions and line current. The device is passive-there are no motors or electronic controls. The actuator (which uses Shape Memory Alloy) is thermally-affected such that its length decreases as a function of line temperature, producing a pull on the line and thereby compensating for sag. As line temperature returns to normal, SnulrtConductorm returns to its original length and therefore automatically resets itself.
SmartConductorm can be fitted in-line or between a fixed point on the tower and a power line suspended from the tower or as a complete replacement of an existing line or in a new instsilation.

EXAMPI.E 1: SLiM'CM
In one example of SLiW, as shown in FIGS. 2 and 4, the SMA element (actuator) is composed of a Nickel-Titanium alloy (also called NiTi or Nitinol). The actuator element is about two inches in diameter and three feet long and is made from about 80 NiTi wires, each having a diameter of about 1/8 inches. The ends of the wires are swaged into two compression flttings, which hold the wires parallel and trazisrnit force and current to the wires. The body of the device is a pipe (a tubular housing) which transmits force and current, and which provides a ftclcnam at one end, and which additionally reduces the corona emitted by the device. A lever, which pivots on the body, magnifies the length change in the SMA wir+es by about 5.5:1 in this example. The current required to heat the SMA wires through their transformation is passed through the wires. In this example, about one-third. of the current is made to pass through the SMA element and the other two-thirds passes through the body of the device.
FIG. 4 shows the components of the SLiMr" example, including the pipe 6 and the actuator assembly 8. At one end of the pipe 6 is the lever assembly 10, composite bearings 12 and an eye connector assembly 14. At an opposite end of the pipe 6 is a shaft 16, composite ttuvst bearings 18 and a clevis assembly 20. FIG. 6 shows the actuator assembly 8 having swage sockets 22 at opposite ends. A first rod end 24 having a spherical bearing 26 press fit into it extends from one of the sockets while a second rod end 28 press fitted with a journal bearing 30 extends from the other.

Note that this is merely an illustrative example, and different dimensions, geometries and materials may be used. For example, the SMA element (actuator) may be composed of any suitable material such as Nickel/Titanium alloys, Copper/Zinc/Aluminium Alloys, or Copper/Aluminium/Nickel Alloys. The actuator element may be about 0.25, 0.5, 1, 3, 5, or up to 8 inches in diameter. The length may be from 6 inches to 100 feet long, depending on the desired application, and for example may be about 1, 3, 5, 9,12, 20, 40 or 80 feet long.

It may be made from a bundle of wires which could include from one to several hundred wires, for example 1, 10, 30, 60, 80,120, 200 or 300 wires may be used. Each wire may have a diameter of about 0.01 to about 2 inches.

EXAMPLE 2: SmartConductorTm In one example of SmartConductorm as shown in FIG. 3, the SMA element is a Nickel-Titanium alloy cable. The cable is about 5/8 inches in diameter and is made from about 19 NiTi wires, each having a diameter of about 1/8 inch, The ends of the wires are swaged into compression fittings, which transmit force to the wires and provide a connection to the conductor. The SMA element is wrapped inside 51 aluminum strands. The ends of the strands are held in compression fittings, which are electrically connected to the conductor and allow for sliding over the NiTi compression fitting. Cutrent passes through the aluminum stcanding and heats the SMA wires via conduction. Tbis is sufficient to cause the required transition in the SMA.
FIG. 5 shows the components of the SmartConductorT'" example including the shape memory wire 32 and aluminum stranding 34. At each end is provided a tube to flat connector 36 and a closed swage socket 38. A slider 40 is also provided at one end.

Once again, this is merely an illustrative example, and the dimensions may vary, and may include any of the dimensions mentioned above for SLiMr"'.

EXAMPLE 3: Test Procedures and Dsta The objective of the functionality test was to verify that the SLiM''4 and Sm.ardConductor'm reduce excess sag as designed when operated on a live transmission line.

In the present test the SLiM7h" device reduces the effective conductor length by approximately 6 inches, and the SmattConductor'M reduces efffective line length by about 2 inches. Extensive research and development has resulted in two full-scale prototype devices -- a SLiMTM prototype and a SmartConductorT~*' prototype. Proper function of these devices had been extensively investigated in a controlled and laboratory environment at Material Integrity Solutions' facilities. Field testing was completed by measuring the difference in sag between a test and control span, then comparing the difference to predicted values. The test was conducted on a 500 ft. span of 795kemi154/7ACSR
(condor) conductor. A test and control span were strung to the same initial tension, then heated by applying current. Sag was measured throughout the test on both spans.

Details of Test Line tensions were measured on each span using 10K lb. S-load cells (Totalcomp, Fair Lawn, N.J., Model TS-10.K-SS). Three !-type thermocouples measured conductor temperature on each span, three additional thermocouples recorded ambient temperatures along the spans, five tbermocouples measured temperatures on the SLiMTM device and two thermocouples measured the temperature of the SmartConductor""'. Current was measured using a coil transformer and voltage was monitored at the output of the loading transformer. Load cell, thermocouple, current and voltage signals were all routed, through data logger hubs to a laptop computer where the data was monitored in real time. Wind speed and direction, and humidity were measured using a portable weather station, Data from the weather station was also ported to the laptop where it could be viewed in real time using FreeWX vl.0$. All data channels were logged at one minute intervals.

FIG. 7 shows the set-up for the functionality test including two support arms 42 separated by span Z, the load cells 44, insulator 46, the loading transformer 48, the therntocouples 50, capacitors 52 and the SLiMTm device.

Each device was installed, in turn, on the test span (FIGS. 2&3). Once a device was in place, both the test span and control span were brought up to the starting tension--5000 lbs.
for SLiMff and 2700 lbs. for SmartConductorTm -- and the data acquisition was started.

Current was then ramped up to 1000-1280 amps and maintained until the conductor tempcrature reached about 212° F. (100° C.). Current was then shut off, and the spans were allowed to cool back to ambient. Heights of the test and control spans midway between the poles were measured at the start of heating, periodically during heating, at 212° F., and at the end of cooling.

The increase in sag from the initial condition to 212° F. was calculated for both spans based on the manual measuremaits of midspan heights. The net effect of each tensioning device, or the "sag differential", was taken as the change in sag of the control span minus the change in sag of the test span.

The predicted effect of the SLiMff and SmartConductoi'"' devices were calculated using in house software based on IEEE Std. 738-1993. Measured initial conditions were used as the inputs for the initial condition, and the thernial balance outlined in IEEE Std. 738-1993 was used to predict the change in sag for each span at the final measured temperature. The calculation included a change in span length during heating to account for cross-arm motion. Also, the predicted values for the test span included a reduetion in the effective line length during heating to simulate the effect of the device. The amount of reduction in line length was based on lab tests in which the eontraction of the device was measured as it was heated while under tension.

B,esults of Test The sag differential between the test and control spans during the test of the Kilvfr" was measured at 44 inches (Table 1). This closely matched the predicted value of 46 inches.
Accounting for cross-arm motion due to difference in test and control span tensions during heating, the total differential in sag created by the SLiM"m device would have been 50 inches. During the test of the SmartConductor"", sag differential was measured at 9,6 inches (Table 2) which was very near the predicted value of 9.8 inches.

TABLE 1: Tension, sag and sag differentials for the SLiM""functionality test.
Control Span 'Ibst Span Tension Height' Change in Sag Tension Height* Change in Sag Sag Differential I4leasured (lbs.) (in) (in) (lbs.) (in) (in) (in) Cold 4731 152 - 4808 151 - -Hot 2685 87 65.0 3747 130 21.0 44.0 Tension Sag Change in Sag Tension Sag Change in Sag Sag Differential Predicted (lbs.) (in) (in) (lbs=) (in) Cn) (in) Cold 4731 81.8 - 4808 80.2 - --Hot 2980 130 48.2 4699 82 1.8 46.4 *I;eight from ground at midspan.

TABLE 2: Tension, sag and sag difj"erentials for the SmartConductorr"' functionality test, Control Span llzt Span Tbnsion Height" Change in Sag Tension Height* Change in Sag Sag Differential Measured (lbs.) (in) (in) (lbs.) (in) (in) (in) - ~r-.
Cold 2917 98.6 - 2925 97.5 - -Hot 2073 41.5 57.1 2202 50 47.5 9.6 Tension Sag Change in Sag Tbnsion Sag Change in Sag Sag Differential Predicted (lbs) (in) (in) (lbs.) (in) (in) (in) Cold 2917 135 - 2925 135 - -Hot 2152 184 48.9 2274 174 39.1 9.8 *Height from ground at midspan.

S[TMMARY, RAMMCA'TI4NS AND SCOPE
Accordingly, it will be apparent that the invention will automatically counteraCt the effects of line sag caused by thermal expansion of the line and will allow the building of fewer, shorter towers and will reduce the cost of retrofitting existing towers and will allow the transmission of higher currents during times of peak electricity demand.

The invention provides a means of mitigating power line sag which is considerably less expensive than current means; it provides a means of mitigating power line sag which is automatic and self-adjusting such that the same change in conditions (line current, ambient temperature, wind and solar radiation) that causes the line to sag will concomitantly cause the invention to act to mitigate the line sag; it provides a means of mitigating power line sag without the necessity of retrofitting transmission towers to make them taller, it provides a means of mitigating power line sag which will allow transmission towers to be spaced at greater intervals than is presently necessary, thereby necessitating the erecting of fewer transmission towc.rs; it provides a means of mitigating power line sag which will allow the building of shorter transmission towers than is presently necessary;
it also provides a means of mitigating power line sag without reducing line current (load).
Additionally, the invention is a relatively simple mechanical device, reducing the cost of manufacture and also reducing the probable need for maintenance. The invention is equally applicable to any suspended or hanging line, not just to a line carrying electric power.

The specificities in the above description should not be construed to limit the scope of the invention, but rather as examples of possible prefemed embodiments. The sizes of the components may vary depending on the size of the tower to which the invention is fitted and the size of the voltage being carried in the power line. The materials used to construct the components may be as suggested above or may be any other materials with suitable mechanical and electrical properties. The actuator rod may be made of a number of substances which contract in response to increasing temperature such as metal alloys but may be made from other substances such as composite materials such as carbon fiber, glass fiber, or an electrically non-conducting metal-filled plastic and the like.

Thus, since numerous modifications and alternate embodiments will readily occur to those skilled in the art, the scope of the invention should not be limitied by the aforementioned illustrative embodiments, but should be determined by the appended claims and their equivalents.

Claims (12)

1. For reducing sag in a suspended cable, a sag-compensating device having a first end and a second end and comprising a shape memory alloy actuator disposed therebetween, wherein the actuator contracts as its temperature increases, and wherein the suspended cable is not strung continuously between the first end and the second end, and wherein at least one end is adapted to be connected, directly or indirectly to the suspended cable, and wherein the other end is adapted to be connected, directly or indirectly to either another section of the suspended cable or to a fixed point, wherein as the temperature of the actuator increases, the actuator contracts, and a pulling force is applied to the cable by the actuator, reducing sag in the cable.

2. The device of claim 1 wherein one end of the device is attached to a suspended cable and the other end is connected to a fixed point.

3. The device of claim 2 wherein the fixed point is a tower.

4. The device of claim 1 wherein the cable is a power line that carries a current.

5. The device of claim 4 wherein at least part of the current is conducted through the device.

6. The device of claim 4 wherein at least part of the current is conducted through the actuator.

7. The device of claim 4 wherein the device further comprises a structural element disposed between the first and second end of the device.

S. The device of claim 7 wherein the structural element is a tubular housing having a first end and a second end and wherein the tubular housing substantially surrounds the actuator, and contacts the shape memory alloy via a pivoted contact point at at least one end.

9. The device of claim 7 wherein the pulling force of the actuator is transmitted by at least one lever pivotally attached to the structural element.

10. The device of claim 9 wherein the device employs only a single lever.

11. The device of claim 1 wherein the pulling force of the actuator is transmitted to the line directly, without the use of a lever.

12. The device of claim 1 wherein the shape memory alloy comprises a binary nickel-titanium (NiTi) shape memory alloy.